This invention relates to systems for ascertaining the presence of hydrocarbons in an environmental medium and, more particularly, to improved methods and apparatus using inductive coupling to generate and sense induced current in a conductive environmental medium and thereby detect changes in the conductivity of such medium.
It is well known that hydrocarbons, such as crude oil and natural gas, are being produced in increasing quantities worldwide to supply growing energy needs. However, the uncontrolled release of hydrocarbons into the environment can produce substantial environmental degradation. Accordingly, the production of hydrocarbons is generally done in a controlled system.
One particular environment where increasing quantities of hydrocarbons are being produced is the subsea floor at various offshore locations. This production is being obtained at ever-increasing depths which are generally hostile or inaccessible to production personnel. Accordingly, there is usually provided a surface platform for monitoring and controlling the production at the subsea location and various elements are located on the ocean floor responding to the surface controls. An entire production unit may consist of a central production platform, a subsea gathering station, several central control platforms having wells associated with them and further controlling satellite wells about the control module.
All of the above units must be interconnected by production piping having joints suitable for use in the hostile seawater environment and from which hydrocarbons might escape from the production system. It is also apparent that prompt detection of incipient hydrocarbon leaks is a difficult task in the submerged environment.
Desirably, a plurality of detection units would be provided placed at locations with a likelihood of hydrocarbon escape. Such locations, for example, would be adjacent the control unit attachment to the well casing, adjacent pipe joints, adjacent underwater storage tanks, or around various valving for controlling the flow of hydrocarbons. Desirably, the presence of hydrocarbons in small quantities should produce an output indication which can be transmitted to a surface location to alert production personnel that corrective action is needed.
The high conductivity of sea water has been utilized in prior devices for measuring the salinity of seawater. If a toroidal inductive element is immersed in the sea and electrically excited, a magnetic field is generated within the toroid which produces an electrical current in the adjacent seawater. A second toroidal inductive element may be used to detect the resulting circulating electrical current to produce an output signal which is related to the circulating current in the environmental medium.
Various prior patents teach the use of coupled toroidal inductors to measure water salinity. For example, in U.S. Pat. No. 3,292,077 to Sloughter, the output from the detecting inductive element controls a voltage controlled oscillator and thereby produces a signal of a frequency functionally related to the conductivity of the seawater. In U.S. Pat. No. 3,855,522 to Kobayashi, U.S. Pat. No. 3,510,761 to Brown, and U.S. Pat. No. 3,603,873 to Cirulis, various secondary windings are provided on the torodial elements to obtain improved features such as automatic zeroing, reference signals, and temperature compensating signals. Other U.S. Pat. No. 3,491,287 to Brown and U.S. Pat. No. 3,389,332 to Ketcham, illustrate various improvements in systems using inductive elements for precision measurements of salinity.
The detection of hydrocarbons escaping from a subsea production system does not require a precision measurement of seawater salinity; but does require a system which produces a highly reliable output indication functionally related to the presence or absence of hydrocarbons. This output signal must also be obtained while the system is packaged in the manner necessary for long term reliable operation exposed to the pressure and temperatures associated with a subsea environment and the corrosive effects of the sea water. It is believed that one prior device uses the inductive coupling technique discussed above for salinity measuring apparatus, positioned in an inverted bucket-type housing to determine the presence of hydrocarbons. Escaping hydrocarbons are captured within the housing, displacing the sea water from the housing, changing the conductivity of the environmental medium surrounding the coupled inductive elements, and producing a corresponding change in the voltage induced in the second toroidal inductor. Once a predetermined change in the induced voltage occurs, an alarm is triggered to initiate corrective action.
There are, however, problems with operating the prior art devices in a subsea production system. Frequently the sensors are located at some distance from the central production platform or subsea control module and this necessitates interconnecting signal cables. A particularly suitable connecting cable interconnects with extending portions of cable by using magnetic coupling elements. In this manner, the mechanical difficulties associated with sealing about a plurality of extending conductor elements are avoided. However, significant signal attenuation and/or distortion can occur through these connecting elements. To maintain adequate signal strength for high-reliability hydrocarbon detection it has been necessary to provide for signal amplification and conditioning within the subsea signaling system. But the addition of such active circuits tends, per se, to lessen the overall system reliability. For this as well as other reasons, it is desirable to minimize the number of active components which must be placed in the ocean environment.
Further, it will be appreciated that the signal should remain sufficiently strong to accommodate variations in the attenuation through the cable connectors as a result of varying sea water temperature and pressures and through possible corrosion and fouling associated with the underwater environment. Spurious indications can significantly disrupt production, resulting in extremely expensive and time consuming shutdowns for underwater inspection. Thus, a sensitive and highly reliable sensor system must be provided.
It is desirable to form the toroidal elements with a minimum number of turns in order to maximize the amplitude of the signal across the receiving inductor load. Where both a transmitting and a receiving toroidal element are provided with the same number of turns, N, it can be shown that: EQU V.sub.out =V.sub.excitation [1/(1+N.sup.2 R.sub.m /R.sub.s)]
where
V.sub.out =voltage across receiver load PA1 V.sub.excitation =voltage across transmitter turns PA1 N=number of coil turns on each of transmitter and receiver elements PA1 R.sub.s =equivalent resistance of surrounding environmental medium PA1 R.sub.m =equivalent resistance of receiver load
Thus, the excitation signal is attenuated in inverse proportion to the square of the number of coil turns.
Reducing the number of coil turns, however, reduces the inductive reactance of the transmitter inductive element. A reduced reactance acts to reduce V.sub.exciting since the actual input signal voltage is determined by the ratio of the coil reactance to the total reactance of the transmitter cable and coil.
Also, the magnetic characteristics of the materials forming inductive sensing elements change as a function of temperature. It has been found that the temperature changes are less pronounced for core elements having a low magnetic permeability. Thus, to avoid temperature-dependent signal fluctuations it would be desirable to use these low permeability materials. However, the low permeability core materials further reduce the inductive reactance of the toroidal inductors, which reduces the signal throughput between the transmitter and receiver elements.
As hereinafter discussed, one aspect of the present invention relates to increasing detection sensitivity by providing a resonant circuit in either the transmitting or detecting inductive elements, or both. This requires capacitive elements which are capable of withstanding the hostile environment at the subsea locations. However, relatively low frequencies are used in the underwater signal system to reduce signal attenuation, normally necessitating large values of capacitance to resonate the torodial inductor at appropriate frequencies. Large capacitors having the requisite stability, lifetime, and resistance to subsea pressure and temperature conditions are difficult to obtain and inordinately expensive. Typically, available large capacitors are rolled foil-type capacitors with varying internal void spaces. Subsea pressures act to collapse these voids unless one-atmosphere pressure enclosures are provided. In contrast, smaller capacitors are formed of solid plate-type elements capable of withstanding the pressure.
It will also be appreciated that the hydrocarbon sensor system must be capable of accommodating a variety of temperature and pressure conditions which affect the magnetic characteristics of the toroidal core element and, hence, the inductance of the inductive element. Thus, the resonant frequency of an LC circuit, if provided, will not be a constant value but will vary about some predetermined design value.
These problems are also overcome by the present invention, however, and improved methods and apparatus are provided for reliably monitoring the subsea environment for the presence of escaping hydrocarbons.